(1) Field of the Invention
The present invention relates to a cylinder liner used for reciprocating internal combustion engines, which is excellent in both the consumption performance of lubricating oil and seizure resistance, and a method for producing it.
(2) Background Information
The cylinder liner of reciprocating internal combustion engines such as engines, is a part to which most complex forces are applied in the internal combustion engines, such as impact force, thermal stress or combustion gas pressure along with the sliding of a piston. Accordingly, the cylinder liner is made of a material excellent in e.g. wear resistance and thermal resistance, and particularly the inner face of the cylinder liner is processed so that good lubricating properties and seizure resistance can be obtained.
For example, if the surface roughness of the cylinder liner is made large, the lubricating oil consumption tends to increase, and if made small, seizure of the piston ring and cylinder liner is likely to occur, whereby the surface roughness of the cylinder liner is usually adjusted to a range of from 1.2 to 4.0 .mu.mRz.
On the other hand, along the regulation of exhaust gas in recent years, it has been demanded to further reduce the lubricating oil consumption of internal combustion engines, whereas it has been further strongly demanded to obtain higher output than the past. To cope with such situation, various improvements have been made with respect to the cylinder liner.
In order to improve the lubricating oil consumption performance and seizure resistance, as a technic wherein surface characteristics of a sliding face of a cylinder liner are improved to reduce the lubricating oil consumption, a technic wherein the surface roughness of the inner face at the upper portion of the cylinder liner is made higher than the surface roughness of the inner face at the lower portion (Japanese Unexamined Utility Model Publication No. 60-147736), and a technic wherein the surface roughness of the inner face of a cylinder is reduced at the portion in the vicinity of a top dead center of a top ring (Japanese Patent Publication No. 2-32462), have been known.
However, in the cylinder liners of these conventional technics, the sliding face is entirely a cast-iron face, and the surface roughness is simply partially changed, whereby the improvements in the lubricating oil consumption performance and seizure resistance, are limitative, and accordingly can not meet the demands for the regulation of exhaust gas. Further, with honing process, it is difficult to produce cylinder liners having the surface roughness partially changed on an industrial scale, whereby it has been difficult to realize such cylinder liners.
Under such circumstances, as the ones wherein the above technics have been improved, a technic wherein a phosphate coating is formed on the entire inner face of a cylinder liner (Japanese Unexamined Utility Model Publication No. 50-51052), and further a technic wherein the inner face of a cylinder liner to which a phosphate coating is applied, is shallowly etched (Japanese Utility Model Publication No. 62-26444), have been known.
However, the phosphate coating is composed of a chemical conversion layer, and at its surface, a crystal layer of a phosphate is formed, and below the surface, etching pockets are formed by local dissolution of a base material. In the etching pockets, a so-called amorphous layer which is excellent in oil-retaining properties, is formed. However, the crystal layer at the surface has a Mohs hardness of 6 to 7, and is accordingly hard and brittle, whereby there is a problem that the crystal surface tends to hurt metals and a piston made of a soft metal. Accordingly, Japanese Patent Publication No. 57-61954 discloses technic wherein the crystal layer of a phosphate is mechanically removed and a porous chemical conversion layer (amorphous layer) is allowed to remain in the etching pockets.
The depth of the etching pockets is represented by the surface roughness of the exposed cast-iron face from which the phosphate coating is peeled off with a chromic acid solution, or by a load curve of the surface roughness. In each of Japanese Utility Model Publications No. 57-61954 and No. 62-26444, the surface roughness of the etching pocket portion of the phosphate coating of the cylinder liner, is controlled to be from 3 to 8 .mu.mRz, from the viewpoint of the balance of the durability in the seizure resistance and lubricating oil consumption, and the surface roughness of the remaining phosphate coating after the crystal layer is removed, is usually from 0.5 to 3.2 .mu.mRz.
However, the above technics have the following problems. If a phosphate coating with a shallow etching pocket is formed in view of the importance of the lubricating oil consumption performance, the seizure resistance at the portion in the vicinity of the top dead center of the top ring tends to be inadequate. On the other hand, if a phosphate coating with a deep etching pocket is formed in view of the importance of the seizure resistance, when the engine is operated, etching pocket portions, graphite portions of cast-iron or perlite matrix portions at the lower portion of the cylinder liner tend to fall off by the sliding of the piston ring, whereby the surface roughness gradually increases and resultingly the lubricating oil consumption will gradually increase.